This invention relates to a process for producing pitch carbon fibers which avoids formation of cracks which run in the axial direction of the fibers.
It is well recognized in the prior art that carbon fibers prepared from pitch can be subject to axial cracking which decreases the fibers' strength, and thus their utility and value. The source of the cracking has been identified as fiber microstructure which is radial in nature rather than either random or "onion skin". See U.S. Pat. No. 4,504,454 for a description and drawings and photos of the cracking phenomenon and various fiber microstructures. There have been several approaches to the resolution of this problem reported in the art. U S. Pat. No. 4,504,454 concentrates on spinning conditions. Other references such as U.S. Pat. No. 4,331,620, U.S. Pat. No. 4,376,747, and U.S. Pat. No. 4,717,331 focus on the placing of inserts in the spinneret which yield modification of pitch flow in the spinneret to produce the desired nonradial microstructure in the fiber. Operation of spinnerets with moving parts on a commercial scale is very difficult. Similarly, maintaining continuity and uniformity of fibers spun from spinnerets having particulate or other very fine porous structures inside the spinneret is a very difficult task on a commercial scale.
Another approach to the problem has been to alter the geometry of the spinneret itself. See for example U.S. Pat. No. 4,576,811 and U.S. Pat. No. 4,628,001, as well as Japanese patent applications Kokai 61(1986)-75820 and 75821, as well as Japanese patent application 168127-1984. The '811 patent maintains a typical spinneret geometry, but examined the effects of various modifications of internal angles :n the zone which joins the counterbore and capillary. The '001 patent describes the use of non-round spinnerets and produces mostly non-round fibers, which may be less desirable for some applications. While strong fibers are produced, including some round small diameter fibers, the use of non-round spinnerets might present manufacturing or operating difficulties. The Japanese applications describe spinnerets which provide variation in cross-sectional area through which the pitch passes. These spinnerets can produce round fibers, but the non-conventional spinneret profile can lead to difficulties in manufacturing the spinnerets, and in cleaning them.
This invention is capable of producing generally round cross-section fibers with spinnerets which are relatively simple to manufacture and maintain. The fibers have high strength due to random microstructure which prevents axial cracking. This is true, even for fibers of large diameters. Strong large diameter continuous carbon fibers have not been available heretofore due to the difficulties in producing such fibers. Accordingly, this invention includes both continuous fibers which are strong and large in diameter, and the process of fiber preparation, which is useful for fibers of both large and small diameters.